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Novel Carbon-Based Adsorbents for Thermal Desorption Applications

Introduction

Volatile organic compound (VOC) atmospheres typically contain a signifi cant amount of moisture. Because the condensation of water on the adsorbent surface can compete with the organic analytes of interest, hydrophobic adsorbents are required. Many consider thermal desorption as the preferred method of VOC extraction (over solvent desorption) because the sample is concentrated and can be analyzed without dilution of the sample. Therefore, adsorbents with high thermal stability and low background levels are required. In the past decade, the use of carbon-based adsorbents has increased due to their superior performance.

The Problem With Porous Polymers

Porous polymers are commonly used for sampling VOCs. However, these materials are plagued with variable background levels. If a porous polymer becomes exposed to oxygen, even at trace levels, during the conditioning and/or desorption processes, oxidation of the polymer framework can produce high background levels. This is caused by the release of residual monomers left in the adsorbent during manufacturing. In addition to high background, small voids could be created as the residual monomers are baked out of the adsorbent. If these voids become large enough, they can lead to channeling during sampling and/or desorption.

The Solution: Carbon-Based Adsorbents

Carbon-based adsorbents offer more choices for sampling atmospheres containing VOCs. These graphitized carbons and carbon molecular sieves are hydrophobic, effi ciently release the compounds of interest during the desorption process, allow a broader spectrum of analytes to be collected, and exhibit lower background levels than porous polymers. Carbon-based adsorbents do not exhibit high baseline problems because they are manufactured at temperatures well above 500 °C. This is hot enough to ensure complete pyrolysis of the polymeric precursor material so that no residual monomers remain. Therefore, there is nothing left to be oxidized, no voids can be created, and channeling is not a concern.

Hydrophobicity Test

To create a dynamic humid atmosphere, a stream of “dry” air was mixed with a stream of “wet” air. The stream of “wet” air was made by bubbling clean air through deionized water. By mixing the two gas streams in the correct proportion, a humidifi ed stream with a relative humidity level of 85% RH was created. The total fl ow was 0.05 L/min. A pre-conditioned tube was weighed to get its tare weight and then attached to the outlet of the humidifi ed gas stream. After 1 liter of humidifi ed air passed through the tube, it was removed and weighed. This was repeated four more times until 5 liters passed through the tube. As shown in Table 1, the carbon-based adsorbents did not retain signifi cant amounts of water. The amount of water exposed to the tubes is shown for comparison.

Table 1. Hydrophobic Characteristics of Three Carbon- Based Adsorbents (weight gain in grams)

Background Test

In Figure 1, a comparison of the background level is shown for both Tenax® TA and Carbopack X. Before the analysis, each tube was conditioned at 50 mL/min. at 320 °C for 1 hour. After the tubes reached room temperature, they were thermally desorbed to determine their background. The chromatograms were generated using the conditions specifi ed in Figure 1. As shown, the porous polymer Tenax TA exhibits background of some residual benzene ring compounds that are breakdown products of the polymer. In comparison, the carbon-based adsorbent Carbopack X does not exhibit any background peaks, allowing for a better quality analysis.

Conclusion

Today there are novel carbon-based adsorbents that can be used with, or in place of, porous polymers. These carbon-based adsorbents have low background, retain minimal water while sampling humid environments, and can be used to sample a broader spectrum of analytes. Presented in this article is just a small sampling of Supelco’s continued research and development on the most state-of-the-art carbon-based adsorbents.